In the case of producing a support for photographic paper, it is necessary to apply a waterproofing treatment for preventing water and processing chemicals from permeating into the inside of the paper. For this purpose, the procedure that has generally been employed is to overcoat the paper with a polymer covering layer, for example, a polyolefin such as polyethylene. However, in order to improve the production efficiency and reduce the thickness of the coating layer, in order to reduce the cost in the foregoing method, it has been necessary to perform the coating by melting the polyolefin at about 300.degree. C., and hence the polyolefin has a tendency to be thermally decomposed, which results in the occurrence of yellowing and the formation of pin holes in the coated layer. Also, in the above-described method, the dispersibility of a white pigment, which is incorporated in the polyolefin for improving the covering power, is poor, whereby the content of the white pigment is limited. In general, for example, titanium dioxide cannot be used in an amount larger than about 20% by weight. The photographic images obtained by using such a support are frequently insufficient in sharpness.
The reason for this seems to be that when the concentration of a white pigment in the coated polyolefin layer is relatively low, only a small portion of the incident light is reflected at the surface of the polyolefin layer during the light-exposure of a photographic material including said support, with the greater part of the light being scattered from the pigment particles not at the surface of the polyolefin layer, and the resulting secondary light exposure by the scattered light from said particles not at the surface reduces the sharpness of photographic images.
For overcoming the above-described problem concerning the surface structure of the support prepared by melt-coating a polyolefin containing a white pigment on a paper and the sharpness of images obtained therewith, various improvements have been proposed. For example, Japanese Patent Application (OPI) Nos. 27257/82, corresponding to U.S. Pat. No. 4,384,040, and 30830/82, corresponding to U.S. Pat. No. 4,364,971, (the term "OPI" indicates an unexamined published patent application open to public inspection) disclose a method involving coating a composition containing an organic compound having an unsaturated bond capable of being polymerized by electron beam irradiation and an inorganic white pigment on a paper support and hardening the coated layer by the irradiation of electron beam. According to the method, a waterproof paper support for photographic papers having a high content of white pigment can be obtained and hence the photographic paper having such a paper support can provide photographic images having high sharpness. Also, Japanese Patent Application (OPI) No. 30830/82 discloses a method for producing a paper support for photographic papers by coating a mixture of an organic compound capable of being hardened by irradiation with electron beam and a white pigment on a base paper, pressing the coated layer onto a high-gloss face under weak pressure, irradiating the coated layer from the back side of the paper by accelerated electron beam while maintaining a contact state of the coated layer with the high-gloss face to harden the coated layer, and then separating the paper support from the high-gloss face. By this method, a support superior in flatness and gloss to conventional polyolefin-coated paper support for photographic paper can be obtained.
Furthermore, Japanese Patent Application (OPI) No. 120934/82 discloses a method for producing a paper support for photographic paper having plastic coatings on both surfaces. The method comprises coating a polyethylene backed base paper with a mixture of an organic compound capable of being hardened by irradiation with electron beam, hardening the coated layer by the irradiation of electron beam, and forming thereon plastic layers. The paper support for photographic paper obtained by this method has good flatness and does not have adverse influences on the silver halide photographic emulsion layers formed thereon, and photographic paper having such a support gives photographic images having good quality.
However, since in these methods the electron beam are applied after coating a mixture of an organic compound capable of being hardened by the irradiation of electron beam and a white pigment on a base paper or a polyolefincoated paper, the base paper tends to be yellowed by the action of electron beam, thus reducing the whiteness and quality of photographic images formed on the paper support. Furthermore, the yellowing of the base paper further proceeds with the passage of time after the irradiation the electron beam.